xe2x80x9cTotal storage capacityxe2x80x9d (TSC), xe2x80x9cabsolute stored chargexe2x80x9d (ASC), xe2x80x9cstate-of-chargexe2x80x9d (SOC), xe2x80x9cabsolute cranking currentxe2x80x9d (ACC), xe2x80x9cfully charged cranking currentxe2x80x9d (FCCC) and xe2x80x9cstate-of-healthxe2x80x9d (SOH) are important performance parameters of an electrochemical cell/battery. These six parameters are assumed herein to have the following definitions:
xe2x80x9cTotal storage capacityxe2x80x9d (TSC) denotes the total amount of charge that a fully charged battery can supply under specified discharge conditions. TSC is usually expressed in ampere-hours or in reserve capacity minutes.
xe2x80x9cAbsolute stored chargexe2x80x9d (ASC)xe2x80x94also expressed in ampere-hours or reserve capacity minutesxe2x80x94denotes the amount of charge that a battery can supply in its current charge state. As a battery is discharged, its ASC decreasesxe2x80x94much like the level of liquid in a fuel tank.
xe2x80x9cState-of-chargexe2x80x9d (SOC), or xe2x80x9crelative stored chargexe2x80x9d, is the ratio of a battery""s ASC to its TSCxe2x80x94generally expressed as a percentage. A battery""s SOC indicates whether charging is advisable and identifies the point at which charging should be discontinued.
xe2x80x9cAbsolute cranking currentxe2x80x9d (ACC) denotes the high-rate discharge current in amperes that a battery can sustain at a specified voltage for a specified time in its present charge state. As a battery discharges, its ACC decreases.
xe2x80x9cFully charged cranking currentxe2x80x9d (FCCC) denotes the value that the ACC would assume if the battery were fully charged.
xe2x80x9cState-of-healthxe2x80x9d (SOH) describes a battery""s full charge capability, either its TSC or its FCCC, vis-à-vis its rated specifications. SOH identifies the point at which battery replacement is advisable.
Both ASC and TSC have traditionally been measured by performing timed-discharge tests on batteries that are partially or fully charged, respectively. Because of the time and expense involved in performing complete discharge tests, other techniques for determining ASC and TSC have been proposed. In U.S. Pat. No. 6,255,801, Chalasani claims to determine battery capacity from observations of the coup de fouet effect. O""Sullivan, in U.S. Pat. No. 6,211,654, discloses a method for predicting battery capacity from the discharge characteristics over a relatively short time period at the beginning of a full discharge.
Techniques employing time-varying signals have also been proposed. Sharaf, in U.S. Pat. No. 3,808,522, purportedly determines the ampere-hour capacity of a lead-acid battery from ac measurements of its internal resistance. Yang, in U.S. Pat. No. 5,126,675, also uses single-frequency internal resistance measurements to predict battery capacity. Muramatsu reports in U.S. Pat. No. 4,678,998 that he can determine both the remaining amp-hour capacity and the remaining service life of a battery from measurements of the magnitude of the ac impedance at two different frequencies. Fang, in U.S. Pat. No. 5,241,275, teaches a method for determining remaining capacity from complex impedance measured at two or three frequencies in the range from 0.001 to 1.0 Hz. Hampson, et al., in U.K. Patent Application GB 2,175,700A, report determining battery capacity from the frequency of the maximum value of capacitive reactance in the xe2x80x9cimpedance characteristic curvexe2x80x9d. Yoon et al., in U.S. Pat. Nos. 6,208,147 and 6,160,382, claim that a battery""s capacity can be found by analyzing the complete impedance spectrum over a wide frequency range. Presumably, any of these techniques, if effective, could also be used to determine SOH by comparing the TSC thus determined with a rated value.
Champlin, in U.S. Pat. No. 5,140,269, shows that the percent capacity of a standby batteryxe2x80x94and hence its SOHxe2x80x94can be determined from its ac conductance measured at a single frequency if the ac conductance of a reference, fully charged, identically constructed, new battery is known. This method, although quite effective, requires that such ac conductance data be available, apriori.
xe2x80x9cAbsolute cranking currentxe2x80x9d (ACC) and xe2x80x9cfully charged cranking currentxe2x80x9d (FCCC) have been traditionally measured with timed, high-rate, discharge tests. Such tests have many disadvantages, however. They require heavy and cumbersome equipment, cause dangerous sparking, give imprecise results, and leave the battery in a significantly worse condition than existed before the test was performed. In response to the need for a better method, Champlin pioneered a testing technique based upon single-frequency ac conductance measurements. Various aspects of this well-accepted methodology have been disclosed in U.S. Pat. Nos. 3,873,911, 3,909,708, 4,816,768, 4,825,170, 4,881,038, 4,912,416, 5,572,136, 5,585,728, 5,598,098, and 5,821,756.
With lead-acid batteries, SOC has been traditionally evaluated by observing the battery""s open-circuit voltage or the specific gravity of its electrolyte. However, neither of these quantities provides information about the battery""s TSC, ASC, ACC, FCCC, or SOH. Furthermore, specific gravity measurements are messy and impossible to perform on sealed cells. Moreover, open-circuit voltage cannot be measured under load conditions and, at any rate, is imprecisely related to SOC because both xe2x80x9csurface chargexe2x80x9d and temperature affect it.
Because of these drawbacks, several techniques for correcting voltage of lead-acid batteries to obtain SOC have been proposed. These include techniques described by Christianson et al. in U.S. Pat. No. 3,946,299, by Reni et al. in U.S. Pat. No. 5,352,968, and by Hirzel in U.S. Pat. No. 5,381,096. However, such voltage correction methods are not very accurate. Furthermore, they are of little help with electrochemical systems other than lead-acid in which voltage may bear little relationship to SOC.
Due to these and other problems, techniques based upon ac or time-varying signals have been proposed for determining SOC. For example, Latner claims to determine SOC of NiCd batteries from ac bridge measurements of farad capacitance in U.S. Pat. No. 3,562,634. U.S. Pat. No. 3,984,762 to Dowgiallo purports to determine SOC from the phase angle of the complex impedance at a single frequency. In U.S. Pat. No. 4,743,855, Randin et al. assert that SOC can be determined from the argument (i.e., phase angle) of the difference between complex impedances measured at two different frequencies. Bounaga, in U.S. Pat. No. 5,650,937, reportedly determines SOC from measurements of the imaginary part of the complex impedance at a single frequency. Basell et al. purport to determine SOC from the rate of change of impedance with frequency in U.S. Pat. No. 5,717,336. Ding et al., in U.S. Pat. No. 6,094,033, broadly assert that SOC can be determined from a battery""s xe2x80x9cimpedance response, which can include series and parallel equivalent circuit parameters, i.e., resistance, capacitance, and phase angle, among othersxe2x80x9d. Finally, techniques purporting to determine SOC from the transient response to an applied pulsed voltage and/or current are disclosed by Andrieu and Poignant in U.S. Pat. No. 5,530,361 and by Simon in French Patent Application FR 2,749,396A. The fact that none of these methods has gained wide acceptance, however, suggests that they may not be altogether satisfactory methods for determining SOC.
Testing apparatus senses the time-varying electrical response of an electrochemical cell/battery to time-varying electrical excitation. The cell/battery may, or may not, be in service. Computation circuitry responsive to the time-varying electrical response evaluates elements of a unique circuit model representation of the cell/battery. Performance parameters and physical parameters are computed from these element values. Computed performance parameters include, but are not limited to, xe2x80x9ctotal storage capacityxe2x80x9d, xe2x80x9cabsolute stored chargexe2x80x9d, xe2x80x9cstate-of-chargexe2x80x9d, xe2x80x9cabsolute cranking currentxe2x80x9d, xe2x80x9cfully charged cranking currentxe2x80x9d, and xe2x80x9cstate-of-healthxe2x80x9d. Computed physical parameters include, but are not limited to, xe2x80x9cexchange currentxe2x80x9d, xe2x80x9cmaximum exchange currentxe2x80x9d, xe2x80x9ccharge transfer conductancexe2x80x9d, xe2x80x9cmaximum charge transfer conductancexe2x80x9d, xe2x80x9cdouble layer capacitancexe2x80x9d, and xe2x80x9cmaximum double layer capacitancexe2x80x9d. Computed parameters are either displayed to the user, employed to initiate an alarm, or used to control a process such as charging the cell/battery.